Shares in Trinity Mirror, publisher of the Daily Mirror, Sunday Mirror and the People, were down 9p to 62.75p – a 12.5% dip – at 9am on Tuesday.

Last week, Trinity Mirror's shares hit 75.25p, their highest point since February 2011, after new chief executive Simon Fox announced a restructure to merge its national and regional newspaper operations.

The newspaper's share price tumbled after it emerged late on Monday that four civil claims have been filed at the high court against Mirror Group Newspapers (MGN), the Trinity Mirror subsidiary that publishes the company's three national titles.

Civil actions against the publisher have been launched by former England manager Sven-Goran Eriksson; former footballer Garry Flitcroft; actor Shobna Gulati, who played Sunita Alahan in Coronation Street and Anita in Dinnerladies; and Abbie Gibson, the former nanny to David and Victoria Beckham's children.

Eriksson's claim relates to the Daily Mirror when Piers Morgan was editor. The claims lodged on behalf of Gulati, Gibson and Flitcroft, allege phone hacking at either the Sunday Mirror or the People.

These are the first civil claims for phone hacking damages filed against any newspaper publisher other than Rupert Murdoch's News International.

The four claims accuse the newspapers of a "breach of confidence and misuse of private information" relating to the "interception and/or misuse of mobile-phone voicemail messages and/or the interception of telephone accounts".

The claims were filed by the solicitor Mark Lewis, one of the leading lawyers acting for alleged victims of News of the World phone hacking.

A spokesman for MGN said: "We note the allegations made against us by Mark Lewis in today's papers. We have not yet received any claims nor have we been provided with any substantiation for those claims.

"As we have previously stated, all our journalists work within the criminal law and the Press Complaints Commission code of conduct."

Trinity Mirror has always robustly defended itself against allegations of phone hacking at its titles. Sly Bailey, the former chief executive, told the Leveson inquiry in January that it was unhealthy for a company to investigate unsubstantiated allegations about itself.

Bailey said: "I don't think it's a way to conduct a healthy organisation to go around conducting investigations when there's no evidence that our journalists have been involved in phone hacking.

"There was no evidence and we saw no reason to investigate. We have only seen unsubstantiated allegations and I have seen no evidence that phone hacking has ever taken place at Trinity Mirror."

Trinity Mirror carried out a review of its editorial "controls and procedures" when the News of the World phone-hacking scandal was at its height in July 2011.

Morgan, now a CNN talkshow host in the US, edited the Daily Mirror between 1995 and 2004. He gave evidence to the Leveson inquiry in December when he repeatedly denied any knowledge of illegal newsgathering techniques at the tabloid.

But in May, BBC Newsnight presenter Jeremy Paxman told the inquiry that Morgan had personally shown him how to illicitly intercept voicemail messages at a lunch in September 2002.

Paxman claimed that at the same lunch, Morgan had teased Ulrika Jonsson about the details of a private conversation she had had with Eriksson, who was England manager at the time.

In one testy exchange with Robert Jay, the senior counsel to the Leveson inquiry, in December 2011, Morgan said: "Not a single person has made any formal or legal complaint against the Daily Mirror for phone hacking."

Former Blackburn Rovers footballer Flitcroft told the Leveson inquiry in November that he had been hounded by tabloid newspapers over an extra-marital affair in 2001.

Lewis confirmed to the Guardian that the civil claims had been lodged, but said they had not yet been served on MGN. He added that he did not expect to file any further claims against the Daily Mirror's publisher this week.

Lewis said no particulars of claim had been filed, but that relevant dates relating to the alleged activity were submitted to the high court. The individuals now have four months to serve particulars of claims on MGN. The merits of the claim remain to be tested.

The formal hacking allegations come weeks before Leveson is expected to outline a critical assessment of the ethics and practices of the press in his report to prime minister David Cameron on the future of newspaper regulation.

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